1. Field of the Invention
The present invention relates to a zoom lens system and, in particular, relates to an inner-focusing type zoom lens system with a wide angle of view suitable for a single lens reflex camera and the like.
2. Description of Related Art
A number of zoom lens systems consisting of, in order from an object side, a negative lens group and a positive lens group have been proposed. However, there has been no proposals for super-wide zoom lens systems having an angle of view of more than 100xc2x0 and a zoom ratio of 1.7 or more. For example, Japanese Patent Post-Exam Publication No. 60-34734 proposes a wide zoom lens system consisting of two lens groups, a negative lens group and a positive lens group. Although the zoom lens system disclosed in Japanese Patent Post-Exam Publication No. 60-34734 satisfactory corrects distortion by using an aspherical surface, the maximum angle of view is about 86xc2x0 and the zoom ratio is also less than 1.5. Moreover, when the focusing operation is carried out by moving the first lens group, the principal ray passes away from the optical axis with focusing on near object, so that the diameter of the first lens group becomes large in order to keep sufficient light quantity. Further, when the diameter becomes large, it causes a problem that the lens becomes heavy. Furthermore, when the focusing operation is carried out by the first lens group that is relatively heavy, it causes another problem that the focusing speed while performing auto-focusing becomes slow in comparison with a inner-focusing type or a rear-focusing type.
Accordingly, a wide zoom lens system with inner focusing method consisting of a negative lens group and a positive lens group has been proposed in, for example, Japanese Laid-Open Patent Application Nos. 8-327907 and 4-15612. In the zoom lens system disclosed in Japanese Laid-Open Patent Application No. 8-327907, although the zoom ratio is about two, the angle of view is only 83xc2x0. Moreover, in the zoom lens system disclosed in Japanese Laid-Open Patent Application No. 4-15612, although the angle of view is 112.7xc2x0, the zoom ratio is only about 1.4.
The present invention is made in view of the aforementioned problems and has an object to provide an inner-focusing type zoom lens system consisting of two lens groups, a negative lens group and a positive lens group, with the maximum angle of view of more than 100xc2x0 and the zoom ratio of 1.7 or more.
According to one aspect of the present invention, a zoom lens system includes, in order from the object side, a first lens group having a negative refractive power, and a second lens group having a positive refractive power. Zooming operation is carried out by changing the interval between the first lens group G1 and the second lens group G2. The second lens group includes, in order from the object side, a front second lens group having a positive refractive power, and a rear second lens group a having positive refractive power. Focusing operation is carried out by moving the front second group to the image side. The following conditional expression is satisfied;
0.002 less than xcexa3{1/(nixc2x7fi)} less than 0.004 
where ni denotes refractive index for d-line (xcex=587.56 nm) of the i-th lens element counted from the object side, fi denotes the focal length for d-line (xcex=587.56 nm) of the i-th lens element counted from the object side, and the summation is to be carried out from the first lens surface to the last lens surface.
In one preferred embodiment of the present invention, the following conditional expression is satisfied;
1.3 less than f21/f22 less than 1.5 
where f21 denotes the focal length of the front second lens group, and f22 denotes the focal length of the rear second lens group.
In one preferred embodiment of the present invention, the first lens group includes at least three negative lenses and at least one positive lens. A surface of the aforementioned at least three negative lenses is an aspherical surface satisfying the following conditional expression:
2 less than X(14)xe2x88x92X(7) less than 2.5 
where X(y) is denoted by the following aspherical expression:       X    ⁡          (      y      )        =                    (                              y            2                    /          r                )            /              [                  1          +                                    {                              1                -                                                      κ                    ⁡                                          (                                              y                        /                        r                                            )                                                        2                                            }                                      1              /              2                                      ]              +                  C        2            ⁢              y        2              +                  C        4            ⁢              y        4              +                  C        6            ⁢              y        6              +                  C        8            ⁢              y        8              +                  C        10            ⁢              y        10            
where X(14) denotes the displacement along the optical axis from the osculating (tangential) plane at the vertex of the aspherical surface denoted by the above aspherical expression at the height y=14 mm, X(7) denotes the displacement along the optical axis from the osculating plane at the vertex of the aspherical surface denoted by the above aspherical expression at the height y=7 mm, r denotes a paraxial radius of curvature, xcexa denotes the conical coefficient, and Cn denotes the aspherical surface coefficient of n-th order.
According to one aspect of the present invention, a zoom lens system includes, in order from the object side, a first lens group having a negative refractive powers and a second lens group having a positive refractive power. Zooming operation is carried out by changing the interval between the first lens group G1 and the second lens group G2. The second lens group includes, in order from the object side, a front second lens group having a positive refractive power, and a rear second lens group having a positive refractive power. Focusing operation is carried out by moving the front second group to the image side. The following conditional expression is satisfied:
1.3 less than f21/f22 less than 1.5 
where f21 denotes the focal length of the front second lens group, and f22 denotes the focal length of the rear second lens group.
According to one aspect of the present invention, a zoom lens system includes, in order from the object side, a first lens group having negative refractive power, and a second lens group having positive refractive power. Zooming operation is carried out by changing the interval between the first lens group G1 and the second lens group G2. The zoom lens system has a maximum angle of view of 100xc2x0 or more of a zooming range. The second lens group includes, in order from the object side, a front second lens group having positive refractive power and a rear second lens group having positive refractive power. Focusing operation is carried out by moving the front second group to the image side.
In one preferred embodiment of the present invention, the zoom lens system has the zoom ratio of 1.7 or more.
Further, it is preferably that the zoom lens system has the zoom ratio of 1.8 or more.